Remove `head' socket re-locking in sonewconn().

Remove `head' socket re-locking in sonewconn().

uipc_attach() releases solock() because it should be taken after
`unp_gc_lock' rwlock(9) which protects the `unp_link' list. For this
reason, the listening `head' socket should be unlocked too while
sonewconn() calls uipc_attach(). This could be reworked because now
`so_rcv' sockbuf relies on `sb_mtx' mutex(9).

The last one `unp_link' foreach loop within unp_gc() discards sockets
previously marked as UNP_GCDEAD. These sockets are not accessed from the
userland. The only exception is the sosend() threads of connected
sending peers, but they only sbappend*() mbuf(9) to `so_rcv'. So it's
enough to unlink mbuf(9) chain with `sb_mtx' held and discard lockless.

Please note, the existing SS_NEWCONN_WAIT logic was never used because
the listening unix(4) socket protected from concurrent unp_detach() by
vnode(9) lock, however `head' re-locked all times.

ok bluhm